The present invention is concerned with the sweetening, in a fixed bed, of petroleum cuts by catalytic oxidation into disulfides of the mercaptans which they contain.
Theoretically, oxidation of this kind can be carried out easily by mixing the petroleum cut to be treated and an aqueous solution of an alkaline base, such as sodium, wherein a catalyst with a metal chelate base is added, in the presence of an oxidizing agent. The petroleum cut and the aqueous solution of the alkaline base are not miscible. The successive stages in the transformation of the mercaptans into disulfides is produced at the interface of the two liquid phases:
transformation of the mercaptans into mercaptides according to the reaction : EQU RSH+B.sup.+ OH.sup.- .fwdarw.RS.sup.- B.sup.+ +H.sub.2 O
where B.sup.+ is the cation of the base under consideration (Na.sup.+ or K.sup.+, for example),
oxidation of the mercaptides and their transformation into disulfides according to the reaction: EQU 2R--S.sup.- B.sup.+ +H.sub.2 O+1/2O.sub.2 .fwdarw.R--S--S--R+2B.sup.+ OH.sup.-
The capacity of the mercaptans to be oxidized and thus their ability to be transformed into disulfides is dependent on their chemical structure. As a general rule, the greater the number of carbon atoms in the aliphatic chain of the mercaptan, the greater its reactivity.
In the case of petroleum cuts containing mercaptans which are difficult to oxidize, particularly if the content of these latter is high (some petrol cuts and kerosene cuts, for example), it is preferable to treat these petroleum cuts with a supported catalyst in the presence of an alkaline base and of an oxidizing agent. A process of this kind is often called a "sweetening process in fixed bed". The alkaline base which is usually used is most frequently sodium in aqueous solution; it is introduced into the reaction medium either continuously or intermittently to maintain the alkaline conditions and the aqueous phase, both of which are necessary for the oxidation reaction. The oxidizing agent which is usually air is mixed with the petroleum cut to be sweetened. The metal chelate used as a catalyst is usually a metal phthalocyanine, such as cobalt phthalocyanine, for example. The reaction is usually carried out at a pressure of between 5 10.sup.5 and 30.10.sup.5 Pascals, at a temperature of between 20.degree. and 70.degree. C. It is well known to the skilled person that when the temperature is above about 70.degree. C., the stability of the catalyst with a metal chelate base is rapidly reduced, causing degradation in the efficiency of the sweetening reaction.
Moreover, it is appropriate to renew the sodium solution which is used up, firstly because of the impurities from the charge which are dissolved in the solution and which make it unsuitable for recycling, and secondly because of the variation in concentration of the base which is reduced due to the water brought by the charge and due to the transformation of the mercaptans into disulfides.
To overcome this problem, a proposal has been made (in particular in the patents FR 2,343,043, U.S. Pat. No. 4,498,978 and U.S. Pat. No. 4,502,949) to suppress the use of aqueous sodium (or aqueous base). However, so that the reaction can take place in the usual way, the active sites of the support must be placed in contact with the mercaptans present in the petroleum charge, which presupposes a homogeneous medium and thus the absence of an aqueous solution. However, apparently the molecules of water already present in the charge and particularly those produced during the reaction promote the appearance on the surface of the catalyst of an aqueous solution, which, if kept beyond a certain threshold causes a reduction in catalytic activity. A proposal has therefore been made either to incorporate a solid desiccant into the support (U.S. Pat. No. 4,498,978), or to resorb this aqueous phase periodically by drying the catalyst using a polar solvent miscible in water, such as an alcohol (FR Patent 2.640.636). However, these solutions, despite being efficient, are inevitably quite expensive to use.
The prior art mentions a large number of supports which can be used to make a catalyst capable of constituting a fixed bed, including:
active carbons obtained by pyrolysis of wood, peat, lignite, bone or various other carbonaceous materials; PA1 clays and natural silicates, such as diatomaceous earth, fuller's earth, kieselguhr, attapulgite, feldspar, montmorillonite, halloysite and kaolin, and PA1 natural or synthesized refractory mineral oxides, such as silica, zirconium oxide, thorium, boron or mixtures thereof. PA1 when the alkali is mainly potassium; PA1 kaliophitite: K.sub.2 O, Al.sub.2 O.sub.3, a SiO.sub.2 (1.8&lt;a&lt;2.4); PA1 the feldsparhold called leucite: K.sub.2 O, Al.sub.2 O.sub.2, a SiO.sub.2 (3.5&lt;a&lt;4.5); PA1 the zeolites of the type: PA1 erionite or offretite: (K, Na, Mg, Ca)O, Al.sub.2 O.sub.3, a SiO.sub.2 (4&lt;a&lt;8); PA1 mazzite or Omega zeolite (W) : (K, Na, Mg, Ca) O, Al.sub.2 O.sub.3, a SiO.sub.2 (4&lt;a&lt;8); PA1 L zeolite: (K, Na)O, Al.sub.2 O.sub.3, a SiO.sub.2 (5&lt;a&lt;8). PA1 when the alkali is sodium: PA1 the amorphous sodium aluminosilicates, the crystalline organization of which cannot be detected by X diffraction analysis, and the Si/Al atomic ratio of which is less than or equal to 5, preferably less than or equal to 3; PA1 sodalite: Na.sub.2 O, Al.sub.2 O.sub.3, a SiO.sub.2 (1.8&lt;a&lt;2.4); PA1 sodalite, PA1 analcime, PA1 natrolite, PA1 mesolite, PA1 thomsonire, PA1 clinoptilolite, PA1 stilbite, PA1 Na-P1 zeolite, PA1 dachiardite, PA1 chabasite, PA1 gmelinite, PA1 cancrinite, PA1 faujasite comprising the synthetic zeolites X and Y, PA1 A zeolite. PA1 1) the petroleum cut to be sweetened is dried beforehand, it can draw in gradually, by dissolving it, water which is present inside the pore structure of the catalyst. Under these conditions, the water content decreases regularly, and can thus decrease below the limit value of 0.1% by weight. PA1 2) Inversely, if the petroleum cut to be sweetened is saturated with water, and in view of the fact that the sweetening reaction is accompanied by the production of one water molecule per molecule of disulfide formed, the water content of the catalyst can increase and attain values above 25% and in particular 40% by weight, at which values the efficiency of the catalyst is impaired.
Significant improvements enabling a partial or complete remedy to the aforementioned problems have been proposed in various patents, as follows: EP 376,744, EP 252,853 and FR 2,651,791. These patents mention the use of solid catalysts constituted of a support containing:
60% to 90% by weight of a mineral matrix, 5 to 35% by weight of pyrolyzed carbon or active carbon, and 0.05 to 10% by weight of metal chelate. These catalysts can work with a water content of between 0.1 and 50% by weight of the support, and they do not require an aqueous alkaline solution to be added continuously. The mineral part of the support can be selected from a group of constituents including aluminas, clays, aluminosilicates and silicates.
These latter catalysts, despite offering some progress compared with prior art catalysts are only really effective with charges which have relatively low contents of mercaptans (&lt;400 ppm approx.). They prove to be ineffective with charges reputed as being difficult, and show a rapid fall (by a few days or a few dozen days) in activity. In view of such unfavorable conditions, it can be necessary to continuously add small amounts of alkaline aqueous solution which adversely affects the process employing catalysts and which results in liquid rejection (sodium solution containing impurities, in particular) which produces the same problems as those already mentioned hereinabove.